The present disclosure relates generally to switched-mode power supplies with primary side control.
Power supplies are necessary for most of electronic products, to convert the energy from grid power lines or batteries into a power source with specifications required for an electronic product. Switched-mode power supply (SMPS), which commonly employs a power switch and an inductive device for power conversion, is superior in view of conversion efficiency and compact product size, and is popularly adopted in the art. A transformer with a primary side winding and a secondary side winding works as an inductive device for isolation-type SMPS.
There are two types of control technologies regarding to isolation-type SMPS: primary side control (PSC) and secondary side control (SSC). SSC directly detects an output terminal powered by the secondary winding and sends the detect result via a photo coupler to a power controller in the primary side, which accordingly controls the current passing through the primary winding, so as to increase or decrease the power stored in the transformer. Opposite to SSC, PSC detects, for example, a reflection voltage on an auxiliary winding in the primary side to accordingly control the current passing through the primary winding, where the reflection voltage is about in proportion to the output voltage in the secondary side. Simply put, SSC performs voltage detection in the secondary side while PSC does in the primary side. PSC might be more effective in cost, because it does not need the large, costly photo coupler that SSC needs. PSC might be more efficient in respect to power conversion, because it lacks the secondary-side detection circuit which constantly consumes power all the time.
FIG. 1 is a SMPS 10 in the art, employing PSC. A bridge rectifier 20 rectifies alternative current grid power lines AC into direct current input power line VIN, which might be of about a constant voltage or have an M-shaped voltage waveform following the voltage variation of the grid power lines AC. A power controller 26 drives, via the GATE node, to periodically turn ON and OFF the power switch 34. When the power switch 34 is turned ON, performing a short circuit, the current passing through the primary winding PRM increases and so does the electric power stored in the transformer. When it is turned OFF, performing an open circuit, the electric power stored in the transformer releases to build up the output power VOUT (for output load 24) and the operation power VCC (for the power controller 26), via the secondary winding SEC and the auxiliary winding AUX, respectively.
Resistors 28 and 30, forming a voltage divider, together detect the voltage drop VAUX across the auxiliary winding AUX to provide feedback voltage VFB at the feedback node FB of the power controller 26. At the time when the power switch 34 is just turned OFF, the voltage drop VAUX is the reflection voltage to the voltage drop across the secondary winding SEC. Based upon the feedback voltage VFB, the power controller 26 builds up a compensation voltage VCOM over a compensation capacitor 32 and accordingly controls the duty cycle of the power switch 34. The current-sense voltage VCS at node CS informs the power controller 26 the amplitude of the current IPRM through the primary winding PRM and the power switch 34.
FIG. 2 demonstrates the gate signal VGATE, the feedback voltage VFB, and the secondary output current ISEC. If the peak value of the secondary output current ISEC and the real discharge time TDIS-R when the secondary winding SEC discharges the stored energy are acquired, both the total electric charge amount and the average current outputted from the secondary winding can be derived, such that the power controller 26 could regulate the maximum average output current from the secondary winding SEC.
Conventional discharge time detection is to detect the timing when the feedback voltage VFB drops across 0V the first time after the power switch is turned OFF (i.e. the gate signal VGATE is 0 in logic). The detection result works as an indicator of the end of an estimated discharge time TDIS-E, which expectedly starts at the time when the gate signal VGATE turns to 0 in logic, as shown in FIG. 2. The estimated discharge time TDIS-E differs with the real discharge time TDIS-R, however, because the secondary winding, in fact, completes discharging before the feedback voltage VFB drops to 0V. This difference, as shown in FIG. 2, could render uncertainty and misjudgment to the output current from the secondary winding SEC. A SMPS employing the convention discharge time detection, as a result, hardly makes the maximum average output current regulation accurately meet a specified target.
In this specification, the apparatuses or devices with the same symbol are the same or similar in respect to functionality, structure, or feature, and their alternatives could be derived by persons skilled in the art based on the disclosed teaching herein. The explanation of these alternatives is omitted for brevity.